Nucleophilic ligand substitution in triply deprotonated tetrapeptide complexes of copper(II) and nickel(II) with 1,10-phenanthroline and 2,2-bipyridine

Ligand substitution of the triply deprotonated tetrapeptide ligand with bulky α-carbon substituents, in the tetrapeptide complexes of Cu(II) and Ni(II) by the bidentate ligands 2,2-bipyridine and 1,10-phenanthroline has been studied. The mechanism in the Cu^II(H_-3A₄)^2? and the Cu^II(H_-3F₄)^2? complexes shows a proton-assisted nucleophilic attack, and the Cu^II(H_-3V₄)^2? shows both proton-assisted and direct equatorial nucleophilic attack by the bidentate ligands. A factor of ten decrease in the rate of substitution from Cu^II(H_-3A₄)^2? to Cu^II(H_-3V₄)^2?, and also Cu^II(H_-3F₄)^2? is an indication of a steric hindrance on the substitution rate because of atom overcrowding due to the size of the α-carbon substituents in the Cu^II(H_-3V₄)^2? and Cu^II(H_-3F₄)^2? complexes. The substitution of the triply deprotonated tetrapeptide ligand in Ni^II(H_-3A₄)^2? by 2,2-bipyridine and 1,10-phenanthroline shows a kinetic behaviour completely different to that of the Cu(II)-tetrapeptide complexes. Only a direct equatorial nucleophilic attack by the bidentate ligands has been observed.